The proposed research focuses on developing new measures of early child language development and parent input, and using those measures to predict children's literacy skills in kindergarten. The purpose of the mentored phase of the project is to: 1) receive training in measurement models, specifically Item Response Theory, 2) to apply these models to child vocabulary data to develop new measures that focus on rarity of production rather than frequency, 3) to validate these measures, and 4) to model growth of these new measures via training in Hierarchical Linear Modeling (HLM). The independent phase focuses on using the skills learned during the mentored phase to: 1) apply similar models to develop and validate new measures of child syntax and parent input, 2) examine the relation between parent input measures and child vocabulary and syntactic growth, and 3) use the new child language and parent input measures as predictors of child literacy skills in kindergarten. The data come from two existing datasets of transcripts from parent-child interaction. The first dataset is from the Language Development Project at the University of Chicago and consists of 90-minute interactions every 4 months between child ages 14-54 months in a sample of 60 families from diverse backgrounds. The second dataset is from the Early Head Start study at Harvard University and consists of 10-minute interactions once a year between child ages 14-36 months in a sample of 108 low-income families. Both samples have additional data using parent report and standardized measures of child language for validating new measures, and both samples have measures of children's literacy skills at kindergarten for use in the final phase of the research. The overall goals of the research program are: i) to develop measures of children's vocabulary and syntactic skills during early childhood that capture rarity of production, 2) to better understand the relation between specific parent input measures and child language growth, and 3) to further the research on the role of early oral language abilities in predicting later literacy skills. Relevance: Early language skills are a strong predictor of literacy skills and school success. This research will provide us with deeper understanding of children's early language abilities, of the role of parental input in child language development, and of the specific relations between children's early language and later literacy skills. The focus is on low-income children, those most at risk for school failure.